The basic Sindarin adjective for “dark” derived from primitive ✶(n)dūrā from the root √NDU “under, down” (PE17/152), but it acquired an “evil” sense by association with names like Barad-dûr and words like guldur “sorcery” (PE17/31), hence also “gloomy, hellish”. A more neutral word is morn, but strictly speaking that is the colour “black” rather than “dark”.
Conceptual Development: The earliest precursor of this word seems to be ᴱN. drú “dark” from Early Noldorin Word-lists of the 1920s (PE13/142). N. dûr appeared in The Etymologies of the 1930s, but as a derivative of ᴹ√DOƷ “night” rather than ᴹ√NDŪ “go down” (Ety/DOƷ). Later on, S. dûr was only influenced by dû “night” rather than being directly related to it (PE17/152).
_ adj. _dark, gloomy, 'hellish'.